Method and system for processing vascular radiographic images which have been reconstructed by three-dimensional modelling

ABSTRACT

Method and system for processing vascular radiography images which have been reconstructed by three-dimensional modelling, in which: from this three-dimensional modelling there is determined a three-dimensional model known as the masked model which features the calcified elements and the prosthetic elements, but not the vascular elements; a three-dimensional model known as the subtracted model, which features the vascular elements alone, is determined; these two models are merged, weighting their voxels so as to increase the contrast between the images of the masked model and the images of the subtracted model; and summing the voxels thus weighted.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The entire contents of French Patent Application No. 0100737filed Jan. 19, 2001 are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to a method and system forprocessing radiographic images which have been reconstructed bythree-dimensional modelling. In particular, the present invention isapplicable to vascular images which includes an implanted element, suchas a prosthesis.

[0003] There are known numerous three-dimensional imaging methods andsystems which take two-dimensional images obtained, for example, byX-ray fluoroscopy, to produce 3D models of an object, i.e., a patientthat it is desired to be observe. In particular, there are known 3Dangiography systems which, by X-ray fluoroscopy, reconstruct 3D modelsof vessels on which a procedure is to be carried out, for example inorder to treat arterial stenosis. In 3D angiography, there are threecomplementary types of image likely to be obtained, namely:

[0004] a reconstructed model known as a “subtracted” model, indicatingthe vascular elements (“lumens”) alone, without the calcified elementsand the endovascular prostheses;

[0005] a reconstructed model known as a “masked” model, identifying thecalcified elements and the prostheses, but not the vascular elements;and

[0006] a reconstructed model known as an “opacified” model, identifyingthe vascular elements, the calcified elements and the prostheses, butwithout it being very easy to distinguish the various elements, theimage obtained being relatively difficult to interpret.

[0007] Techniques for producing these three types of modelled images,are described in French Patent Application No. 0011486.

BRIEF DESCRIPTION OF THE INVENTION

[0008] An embodiment of the invention is system and method forprocessing a vascular radiography image which has been reconstructed bythree-dimensional modelling comprising:

[0009] (a) determine a three-dimensional model to be known as the maskedmodel which features the calcified elements and an implanted element,but not vascular element;

[0010] (b) determine a three-dimensional model to be known as thesubtracted model, which features the vascular element alone;

[0011] (c) merging the two models and weighting their voxels so as toincrease the contrast between the image of the masked model and theimage of the subtracted model; and

[0012] (d) summing the voxels thus weighted.

BRIEF DESCRIPTION OF THE DRAWING

[0013] An embodiment of the invention will become further apparent fromthe description which follows, which is purely illustrative andnon-limiting and is to be read with reference to the appended drawingsin which:

[0014]FIG. 1 schematically depicts a system for implementing a methodaccording to an embodiment of the invention;

[0015]FIG. 2 is a flow diagram schematically illustrating the varioussteps in the processing in an embodiment of the invention;

[0016]FIG. 3 is an example of a possible curve for one of the functionsused in the processing;

[0017]FIGS. 4a and 4 b illustrate a 3D model and a sectional viewobtained from it, in the case of implementation of a method of oneembodiment of the invention; and

[0018]FIGS. 5a and 5 b illustrate another exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0019] In this embodiment, it is assumed that there is available a setof two-dimensional angiography images obtained around a given anatomicalregion of a patient and from these images it is possible to reconstructthree-dimensional models of the anatomical region and, in particular,subtracted and masked three-dimensional models. These angiography imagesmay, for example, be obtained by x-ray fluoroscopy, etc. The images arestored and processed in a processing unit 5, which is connected tointerface means 6 which, in particular, allow the radiography images tobe displayed (FIG. 1).

[0020] In an embodiment of the processing may be carried out in foursuccessive steps.

[0021] In a first step (step 1 in FIG. 2), the user defines a volumewhich constitutes the region of interest. The region of interest will,in particular, preferably be defined so that it contains only a limitednumber of bones while at the same time fully including the portion orportions of blood vessel the user wishes to view, with any calcifiedelements or prosthesis they may have. In particular, the user mayselect, from a displayed 3D model the vessels to view and for the systemto automatically determine the limits of these vessels. As analternative, the user can make a selection by magnifying the delimitedregions.

[0022] In a second step (step 2), the system automatically reconstructsthe three-dimensional models which correspond to the subtractedreconstruction and the masked reconstruction for the selected region.

[0023] In a third step (step 3), the system performs a 3D mergerprocessing operation on the two models thus obtained.

[0024] And finally, in a fourth step (step 4), the merged 3D image thusobtained is displayed on the interface means 6.

[0025] The merger of the two models is substantially meaningless unlessthe subtracted and masked 3D models have been obtained from the sameangiographic acquisition and correspond to the same 3D frame ofreference.

[0026] The merger processing operation is performed voxel by voxel,calculating the intensity (attenuation coefficient, Mer(v) of a voxel vof the merged image using the formula:${{Mer}(v)} = {{V_{0} \cdot \frac{{Sub}(v)}{{Mean}({Sub})}} + {a \cdot {Y_{b}\left\lbrack {{Mask}(v)} \right\rbrack}}}$

[0027] where:

[0028] v represents the coordinates of the voxel;

[0029] Sub(v) is the intensity (that is to say the attenuationcoefficient) of the voxel v of the subtracted model;

[0030] Mask(v) is the intensity (that is to say the attenuationcoefficient) of the voxel v of the masked 3D model;

[0031] Mean(Sub) is the mean intensity calculated either over the entirevolume considered, or vessel by vessel—using automatic determination ofthe limits of the vessels or vessel portion per vessel portion, oralternatively still, along straight-line portions which constitute themain directions of the vessel;

[0032] V₀ is a predetermined constant which represents the “desired”mean intensity for the depiction of the vessel in the merged image;

[0033] a is a constant of predetermined value which has a value higherthan 1 (for example 10) so as to increase the contrast between thestructures present in the unsubtracted model (vascular prosthesis,calcification, etc.) with respect to the blood vessels;

[0034] Y_(b) is a one-dimensional function which is, for example, alinear straight line, but which may be a more complicated function (c.f.FIG. 3) which depends on a threshold value b and comprises three parts:

[0035] Y_(b) is zero in the interval [0,b];

[0036] Y_(b) varies linearly between b and a value b+b₀, the linearitycoefficient being equal to 1;

[0037] Y_(b) is, in the interval [b+b₀; +∝], a function which increasesless markedly than the linear function of intensity used between b andb+b₀; this function is for example the {square root} function adjustedto ensure continuity of the first-order derivative of the functionY_(b), this third part making it possible to avoid the effects ofsaturation when highly absorbent metal elements are present in themasked image.

[0038] The role of the threshold b is to eliminate the ambient noisepresent in the reconstructed or subtracted model before increasing itscontrast.

[0039] One possible robust automatic estimate of this threshold isobtained as follows:

[0040] calculating a smoothed histogram of the masked model, and doingso over the selected region for treatment, smoothing being achieved bytaking a mean over a smoothing window of a predetermined size and,

[0041] determining the minimum value of the first-order derivative ofthis histogram, the intensity thus obtained being the threshold b.

[0042] Multiplying by the coefficient “a” is advantageously notperformed until after filtering has been performed using the thresholdb.

[0043] Examples of merged images obtained in the way which has just beendescribed are given in FIGS. 4a and 4 b, and in 5 a and 5 b. The imagesin FIGS. 4a and 4 b being, respectively, views in section andperspective revealing calcifications around the vesselsl. The images ofFIGS. 5a and 5 b being, respectively, views in section and perspectiverevealing vascular prosthesis (stents). Merged three-dimensional imagesin which the calcifications and the protheses show up particularlyclearly are thus available. The calcifications or the stents show upparticularly clearly in these images, making them easier to interpret.

[0044] Of course, it is possible for the user, during the procedure, toalter certain parameters and in particular to modify the parameters band a to adjust the contrasts of the various merged parts and controlthe brightness of the unsubtracted images.

[0045] Images thus obtained are similar to those that can be obtained byCT angiography. A two-colour depiction may also be envisaged.

[0046] Various modifications in structure and/or steps and/or functionmay be made by one skilled in the art without departing form the scopeand extent of the invention as recited in the claims.

What is claimed is:
 1. A method for processing radiography imagescomprising: determine from a three-dimensional modelling athree-dimensional model known as the masked model which features acalcified element and an implanted element, but not a vascular element;determine a three-dimensional model known as the subtracted model, whichfeatures the vascular elements alone; merging the two models, weightingtheir voxels so as to increase the contrast between the images of themasked model and the images of the subtracted model; and summing thevoxels thus weighted.
 2. The method according to claim 1, wherein themasked image is filtered by removing therefrom any voxel intensitieswhich are below a given threshold.
 3. The method according to claim 2,wherein the weighting is applied to the voxels after filtering.
 4. Themethod according to claim 1 wherein the voxels of the masked image areweighted by applying to them a weighting law which, over at least onerange of voxel intensities, is a linear function of the intensity. 5.The method according to claim 2 wherein the voxels of the masked imageare weighted by applying to them a weighting law which, over at leastone range of voxel intensities, is a linear function of the intensity.6. The method according to claim 3 wherein the voxels of the maskedimage are weighted by applying to them a weighting law which, over atleast one range of voxels intensities, is a linear function of theintensity.
 7. The method according to claim 4, wherein the voxels of themasked model are weighted by applying to them a weighting law which,outside of the voxel intensity range, increases less markedly than thelinear function of intensity used for the intensity range.
 8. The methodaccording to claim 5, wherein the voxels of the masked model areweighted by applying to them a weighting law which, outside of thevoxels intensity range, increases less markedly than the linear functionof intensity used for the intensity range.
 9. The method according toclaim 6, wherein the voxels of the masked model are weighted by applyingto them a weighting law which, outside of the voxel intensity range,increases less markedly than the linear function of intensity used forthe intensity range.
 10. The method according to claim 7, wherein theweighting law used outside of the intensity range is a function which,give or take a multiplication factor, corresponds to the square rootfunction.
 11. The method according to claim 8, wherein the weighting lawused outside of the intensity range is a function which, give or take amultiplication factor, corresponds to the square root function.
 12. Themethod according to claim 9, wherein the weighting law used outside ofthe intensity range is a function which, give or take a multiplicationfactor, corresponds to the square root function.
 13. The methodaccording to claim 1 wherein the voxels of the subtracted model areweighted by applying to them a coefficient which is the ratio between avalue that corresponds to a desired mean value for the voxels of themodel in the merged model and a mean value that is calculated over thevoxels in the subtracted model.
 14. The method according to claim 13,wherein the mean value is calculated by determining the limits of thevessels or vessel portions and by calculating the mean value in theregion thus determined.
 15. The method according to claim 13, whereinthe mean value is calculated by determining portions of straight lineswhich constitute the main directions of a vessel and by calculating themean value over these straight lines portions.
 16. The method accordingto claim 1 wherein the anatomical region that it is desired to view isselected beforehand, the masked model and the subtracted model and themerged model being determined for the region.
 17. The method accordingto claim 16, wherein the merged model is produced by pointing to theportion or portions of vessels that the user wishes to view andautomatically determining the limits of this or these portion orportions of vessels.
 18. An apparatus for radiographic imagingcomprising: (a) means (5) for providing a three-dimensional model to beknown as the masked model showing a calcified element and an implantedelement, but not a vascular element; (b) means for providing athree-dimensional model to be known as the subtracted model showing thevascular elements alone; (c) means for merging the two models andweighting their voxels so as to increase the contrast between the imageof the masked model and the image of the subtracted model; and (d)summing the voxels thus weighted.
 19. The apparatus according to claim18 comprising: means for filtering the masked image to remove therefromany voxel intensities which are below and given threshold.
 20. Theapparatus according to claim 19 wherein the weighting is applied to thevoxels after filtering.
 21. the apparatus according to claim 18 whereinthe voxels of the masked image are weighted by applying to them aweighting law which, over at least one range of voxel intensities, is alinear function of the intensity.
 22. The apparatus according to claim21 wherein the voxels of the masked model are weighted by applying tothem a weighting law which, outside of the voxel intensity range,increases less markedly than the linear function of intensity used forthe intensity range.
 23. The apparatus according to claim 22 wherein theweighting law used outside of the intensity range is a function which,give or take a multiplication factor, corresponds to the square rootfunction.
 24. The apparatus according to claim 18 wherein the voxels ofthe subtracted meodel are weighted by applying to them a coefficientwhich is the ratio between a value that corresponds to a desired meanvalue for the voxels of the model in the merged model and a mean valuethat is calculated over the voxels in the subtracted model.
 25. Theapparatus according to claim 24 wherein the mean value is calculated bydetermining the limits of the vessels or vessel portions and bycalculating the mean value in the region thus determined.
 26. Theapparatus according to claim 24 wherein the mean value is calculated bydetermining portions of straight lines which constitute the maindirections of a vessel and by calculating the mean value over thesestraight lines portions.
 27. The apparatus according to claim 18 whereinthe anatomical region that it is desired to view is selected beforehand,the masked model and the subtracted model and the merged model beingdetermined for the region.
 28. The apparatus according to claim 27wherein the merged model is produced by pointing to the portion orportions of vessels that the user wishes to view and automaticallydetermining the limits of this or these portion or portions of vessels.